The present invention relates to Class AB amplifiers for internal Low Drop-Out (LDO) circuits. In particular, it relates to improving power supply range of Class AB amplifier output stages.
Class AB amplifiers generally have a complimentary pair of push-pull arrangement where each half of the input signal is individually amplified by one of the push-pull pairs respectively. In some designs, to reduce distortion between the two separate amplified halves, both push-pull pairs are never completely turned off but are designed to have some current flowing even when not amplifying the input signal. The constant powering of the push-pull arrangement reduces distortion in the amplified signal by creating an overlap of the two amplified halves. However, this technique requires a constant supply of power and, therefore, Class AB amplifier designs are generally considered power inefficient.
Along with efficient power consumption, Class AB amplifier designers are concerned with glitch compensation. Glitches represent a sudden change in voltage at an output terminal of an amplifier even though an input voltage remains unchanged. They can occur, for example, when a load impedance at the amplifier's output changes suddenly. In order to compensate for glitches, capacitors can be placed at the amplifier output. However, a capacitor positioned at the amplifier output must be relatively large in order to have enough charge to compensate for glitches thus the capacitor can take up an overwhelming amount of space on the circuit board.
Another option is to position the dominant pole of the amplified signal at an intermediate stage and have a compensating amplifier at the output stage to compensate for glitches. For example, FIG. 1 is a block diagram of a known amplifier system 100. The system 100 includes a first amplifier 110 and a compensating amplifier 120 that provides glitch compensation.
However, compensating amplifiers generally require large power supply voltages to react to glitches. For example, FIG. 2 is a circuit diagram of a known compensating amplifier 120 that requires a large supply voltage VDD. The compensating amplifier 120 includes two push-pull transistors M1, M2 that are both coupled to the output Vout. The compensating amplifier 120 also includes an input transistor M3 that is coupled to the input signal VIntermediate, the output signal Vout, and a current mode amplifier 210. The current mode amplifier 210 includes a diode connected transistor M5 and transistor M4. The current mode amplifier's 210 inputs are coupled to the second push-pull transistor M2 and another diode connected transistor M5′. Since M5 and M5′ are coupled in series, VDD must at least be greater than the sum of each transistor's voltage threshold VTH, which is the voltage required to turn on the transistor, plus some amount of headroom for current source 12. The two cascading diode connected transistors M5, M5′ make the compensating amplifier 120 require a large supply voltage thus limiting the applications of the circuit to high power supplies. Accordingly, there is a need in the art for fast reacting Class AB output stages that operate on low power supplies.